DAY

, a division of time arising from the appearance and disappearance of the sun.

Day is either natural or artificial.

Artificial Day is that which is primarily meant by the word Day, and is the time of its being light, or the time while the sun is above the horizon. Though sometimes the twilight is included in the term daylight; in opposition to night or darkness, being the time from the end of twilight to the beginning of daylight.

Natural Day is the portion of time in which the sun performs one revolution round the earth; or rather the time in which the earth makes a rotation on its axis. And this is either astronomical or civil.

Astronomical Day begins at noon, or when the sun's centre is on the meridian, and is counted 24 hours to the following noon.

Civil Day is the time allotted for day in civil purposes, and begins differently in different nations, but still including one whole rotation of the earth on its axis; beginning either at sun-rise, sun-set, noon, or midnight.

1st, At sun-rising, among the ancient Babylonians, Persians, Syrians, and most other eastern nations, with the present inhabitants of the Balearic islands, the Greeks, &c. 2dly, At sun-setting, among the ancient Athenians and Jews, with the Austrians, Bohemians, Marcomanni, Silesians, modern Italians, and Chinese. 3dly, At noon, with astronomers, and the ancient Umbri and Arabians. And 4thly, at midnight, among the ancient Egyptians and Romans, with the modern English, French, Dutch, Germans, Spaniards, and Portuguese.

The day is divided into hours; and a certain number of days makes a week, a month, or a year.

The different length of the natural day in different climates, has been matter of controversy, viz, whether the natural days be all equally long throughout the year; and if not, what their difference is? A professor of mathematics at Seville, in the Philos. Trans. vol. 10, pa. 425, asserts, from a continued series of observations for three years, that they are all equal. But Mr. Flamsteed, in the same Trans. pa. 429, refutes the opinion; and shews that one day, when the sun is in the equinoctial, is shorter than when he is in the tropics, by 40 seconds; and that 14 tropical days are longer than so many equinoctial ones, by 10 minutes. This inequality of the days flows from two several principles: the one, the eccentricity of the earth's orbit; the other, the obliquity of the ecliptic with regard to the equator, which is the measure of time. As these two causes happen to be differently combined, the length of the day is varied. See Equation of time.

Day's-Work, in Navigation, denotes the reckoning or account of the ship's course, during 24 hours, or between noon and noon.